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Q. How do I add a user to a group under Sun Solaris UNIX operating system from a shell prompt or command line?

A.. You can useadd command to administer a new user login on the UNIX system. Solaris UNIX / HP-UX UNIX uses /etc/passwd, /etc/shadow and /etc/group files to store user information.

Following information is compatible with the Solaris, AIX and HP-UX UNIX operating systems.
Add A New User To A Group

Add a new user called vivek to a group called vsftp, enter:
# useradd -G vsftp -d /data/home/v/vivek -m vivek
# passwd vivek
Where,

* -G vsftp : Add vivek to secondary group called vsftp. This group must exists on the system.
* -d /data/home/v/vivek : Path to the vivek's home directory
* -m : Create the new user's home directory if it does not already exist.
* vivek : Username / login nane

UNIX Add Existing User To A Secondary Group

Use usermod command to modify a user's login definition on the system including group membership. For, example add user vivek to sales group, enter:
# usermod -G sales vivek
Add user tom to sales but also keep his existing membership of ftpusers, enter:
# usermod -G ftpusers,sales tom
UNIX Add Existing User To A Primary Group

Add jerry to sales primary group and change its existing group membership:
# usermod -g sales jerry
For more information read following man pages:
$ man usermod
$ man useradd
$ man passwd

Reference: http://www.cyberciti.biz/faq/unix-add-user-to-group/